Cramping after/ during sex

Hey ladies, 
I'm 27 weeks right now and recently whenever my husband and I have been having sex (which has been ALOT because I'm just so horny all the time!), I experience side cramps. Almost like period cramps but these are a lot more painful. I first felt them when I was on top and I thought it was just because he had grabbed my sides too hard but then later that day when we were having sex again (I told you I was horny lol), I felt it when we were doing "doggie style". 
Normally my pain tolerance is pretty high but these cramps have been so painful that I started sobbing after the first time it happened. It freaked my husband out- he was scared he had hurt me or our baby girl. 
Anyone have any clue what's going on?